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I'm connecting a 13" MacBook Air to a Dell 22" screen using the Apple Mini DisplayPort to DVI Adapter, with an Apple Keyboard with Numeric Keypad plugged into a USB on the MacBook and a USB mouse plugged into the keyboard. Also, the MacBook is running off external power.

When I close the MacBook lid, I thought the Dell monitor would become primary and the MacBook screen real-estate would "go away." However, the MacBook screen remains primary and I can move the mouse pointer off the Dell screen and onto the invisible MacBook screen.

Am I misunderstanding how clamshell mode works?

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Is your mac going into sleep when you close the lid? –  MrDaniel Feb 22 '12 at 19:52
    
@MrDaniel: I'm new to Macs, but I think so. The Apple logo goes dark on the back. This might be a new issue with 10.7.3. –  mikekmcguire Feb 22 '12 at 21:25
    
If the external display remains on and you can move the mouse cursor, the system is probably not going to sleep (a standby state in which you can't interact with OS X). Note that the Apple logo's glow appears to be caused by the LED backlight itself - it turns off whenever the display is off, and isn't an indicator of "sleep" status. –  Dan J Feb 22 '12 at 21:31
    
(In fact, the Apple logo appears to just be a piece of translucent plastic over a hole in the shell: when I first purchased my MacBook Air I was shocked to open the lid and discover that I could light passing through the Apple logo on my display!) –  Dan J Feb 22 '12 at 21:33
    
Is the power light on the front of the Mac Book slowly fading off and on? –  MrDaniel Feb 22 '12 at 21:36
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1 Answer

According to Apple Support, the clamshell mode (lid closed) requires:

  • Power adapter
  • External keyboard, mouse, or trackpad
  • External display

You meet all three criteria. The support article goes on to provide steps for both wired and bluetooth keyboard & mouse combos:

If you are using a wired keyboard and mouse:

  1. Make sure the computer is plugged in to an outlet using the AC power adapter.
  2. Connect a USB keyboard and mouse to your computer.
  3. With the computer turned on connect the Apple portable (using the appropriate Apple adapter if necessary) to the appropriate port on the external display or projector and turn the display or projector on.
  4. After your computer's Desktop appears on the external display, close the computer's lid.
  5. When you close the lid:
    • In OS X Lion, the external display will change to a blue screen, then will show your desktop.
    • In Mac OS X v10.6.8 and earlier, wake the computer up by clicking the mouse button or by pressing a key on your external keyboard. You should now be able to use your Apple portable as you normally would, with a USB keyboard and mouse.

Perhaps you could try opening and closing the lid again.

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1  
I do meet the requirements, and I followed the listed steps -- no clamshell. I tried opening and closing the lid, but no difference. However, I restarted the machine (from MacBook), logged in, shut the lid, and clamshell worked. I assume that I can put the MacBook into sleepmode, then restart it into clamshell mode by tapping the keyboard or moving the mouse (generate a USB event). I'll try that next. –  mikekmcguire Feb 23 '12 at 0:18
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Responding to myself: Manual sleep causes the original issue. On MacRumors there's a discussion of this issue. It looks like the only solution, until Apple addresses the issue, is the manual-restart route or the reinstall method mentioned in the MacRumors link. Unless someone has a different idea, I guess we can mark this as answered? Thanks for all the help and troubleshooting. –  mikekmcguire Feb 23 '12 at 0:37
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For anyone with similar problems, it seems to be confined to the 2011 MacBook Airs, and possibly those that use VGA or DVI outputs (someone reported that HDMI worked as expected). Also, this MacRumers thread claims that the developer preview of Mountain Lion fixes the issue. –  mikekmcguire Feb 23 '12 at 4:28
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